ANATOMICAL DISTRIBUTION OF ARM-MOVEMENT-RELATED NEURONS IN THE PRIMATE SUPERIOR COLLICULUS AND UNDERLYING RETICULAR-FORMATION IN COMPARISONWITH VISUAL AND SACCADIC CELLS

We recorded from 389 ''reach'' neurons (two monkeys) in the superior c olliculus (SC) and underlying reticular formation (RF) or adjacent per iaqueductal grey, whose activity was related to visually guided arm mo vements. Reach neurons were present from approximately 0.7 mm down to a depth of 6 mm below the surface of the SC (mean 3.7+/-1.3, n=389). A lthough this mean distribution was different from that of cells with v isual (mean depth 1.7+/-1.4 mml n=283) or saccadic responses (mean dep th 2.0+/-1.4 mm, n=232), there was a large amount of overlap. Fifty-fi ve per cent of all reach cells (213/389) were assumed to be located in side the SC, The otters were considered to be located in the underlyin g RE The characteristics of visual responses and saccadic bursts (e.g. response latencies, discharge rates, burst durations) of arm-movement -related neurons were not different from those of typical visual or sa ccade cells in the SC. Although reach neurons could be recorded in a l arge area of the SC, they were found more often in the lateral than in the medial parts (chi-squared=19.3, P<0.001). Possible pathways by wh ich arm-movement-related neuronal activity in and below ?he SC might g ain access to spinal motor structures are discussed, The location of a rm-movement-related neurons described in this study is in accordance w ith the known target areas of skeletomotor-related corticotectal proje ctions and with tile sites of origin of tectofugal pathways. It is con cluded that this population of reach cells is in a position to relay a nd transmit limb movement information to the spinal motor system, wher e it may influence (or interact with) motor commands coming from other motor centres.